Speculative Register Reclamation for Power Reduction
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Solution Overview
Problem
The increasing number of in-flight instructions leads to larger physical register files, which consume more power and decrease performance, necessitating efficient release and recovery of physical registers to address these issues.
Innovation Solution
Speculative Register Reclamation (SRR) aggressively releases physical registers upon redefinition of corresponding logical registers, utilizing a payload RAM for data writeback and bypassing, and modifying existing tables and structures like the register alias table and reorder buffer to determine eligibility for early release.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the number of in-flight instructions is increased to enable higher instruction level parallelism, then single-thread performance is improved, but the physical register file size increases leading to higher power consumption and decreased performance
Solution Approach 1:
The patent implements speculative register reclamation that discards physical register allocations when instructions are mis-specified or do not require the registers, and recovers them for reuse. This allows the system to maintain high instruction-level parallelism with fewer physical registers by aggressively reclaiming registers that are no longer needed, thereby reducing power consumption while preserving productivity
Solution Approach 2:
The patent introduces dynamic register allocation and reclamation mechanisms that adaptively manage physical register usage based on actual instruction execution needs. The system dynamically adjusts which registers are kept or released based on speculation outcomes and actual data dependencies, optimizing the balance between maintaining sufficient registers for high ILP and reducing power consumption from excessive register file size
2Productivity
If the physical register file size is increased to support more in-flight instructions, then instruction level parallelism is improved, but device complexity and area increase
Solution Approach 1:
The patent employs speculative reclamation that discards physical register bindings when instructions are mis-specified, and recovers those registers for new allocations. This dynamic discarding and recovering mechanism enables the system to support high instruction-level parallelism temporarily during speculation, then reclaim those registers to reduce the effective register file size and complexity
Solution Approach 2:
The patent uses register alias tables and reorder buffers that maintain virtual mappings and metadata copies rather than requiring all physical registers to be simultaneously active. The system copies register state information in software-managed structures, allowing physical registers to be reused across different instruction contexts, thereby reducing the required physical register file size and complexity
3Use of energy by stationary object
If physical registers are released early upon redefinition to reduce register file size, then power consumption is reduced, but correctness of mis-speculation recovery may be compromised
Solution Approach 1:
The patent implements preliminary checks and validation mechanisms before releasing physical registers. The speculative reclamation logic预先 validates that register release will not compromise mis-speculation recovery by checking dependency chains and speculation outcomes, ensuring that registers are only released when safe to do so, thereby maintaining reliability while reducing power consumption
Solution Approach 2:
The patent uses feedback mechanisms from the reorder buffer and commit logic to monitor instruction execution outcomes and adjust register reclamation decisions accordingly. The system continuously feedbacks on speculation correctness and uses this information to determine whether early register release is safe, ensuring that reliability is maintained while achieving power savings from reduced register file size
Data Source
AI summary
A system determines an original instruction with a first logical register (LR) mapped to a first physical register (PR). The system determines a current instruction with a current LR. A prior instruction is associated with a second LR mapped to a second PR. The system allocates the current LR to a third PR. Responsive to determining that the current and prior instructions are executed in different iterations, the system marks the second PR as not eligible for early release. Responsive to determining that the current LR is previously mapped to the first PR, the allocation comprises a redefinition of the first LR. Responsive to determining that the first PR is eligible for early release and that the current and original instructions are executed in the same or consecutive iterations, the system releases the first PR based upon the redefinition and not the prior instruction completing or the current instruction committing.